The invention relates to an exhaust gas purification system for an internal combustion engine, especially a diesel engine, which is controlled electronically and which includes means for the treatment of the engine exhaust gases by the addition of a reducing agent stored in a storage tank.
For a reduction of the nitric oxides in exhaust gases of a vehicle diesel engine, it is known to add ammonia to the exhaust gases upstream of a catalyzer, two ammonia sensors being provided for detecting the charge level limits in the catalyzer (DE 4,217,552 C1). The addition of ammonia from a storage tank is controlled by means of these sensors, with which the ammonia concentration in the catalyzer is measured. The purification of the exhaust gases is carried out by treatment of the exhaust gases with ammonia which is added in the in the needed amounts and which serves as the reducing agent.
It is known from DE 4,003,515 A1 to reduce the nitric oxides content of exhaust gases of a vehicle diesel engine by the addition of urea as a reducing agent. This process is carried out that is the urea is added to the exhaust gases, in the direction of flow of the exhaust gases, upstream of a zeolite-containing catalyzer.
In any case however, at one point, the reducing agent stored in the tank will be consumed whereupon the NO.sub.2 content in the engine exhaust gas becomes excessively high.
The object of the invention is to provide an exhaust gas purification system of the type described above, which, however, even in the event of a failure of the exhaust gas treatment arrangement, has no inadmissibly high emissions of nitric oxides.